Researchers from King’s College London used a twin study to find that genetic predisposition explained a large amount of the risk for compulsive hoarding — a mental health problem in which people have an overwhelming desire to accumulate items normally considered useless, like old newspapers or junk mail.

Of the more than 5,000 twins in the study, roughly 2 percent showed symptoms of compulsive hoarding and genes appeared to account for half of the variance in risk.

Researcher Dr. David Mataix-Cols said it has long been known that compulsive hoarding tends to run in families.

But he told Reuters Health that what has not been clear is whether that pattern is due to genes or to something in the home environment, like parenting practices.

“Twin studies allow us to separate these two sources,” Mataix-Cols said.

The study, published in the American Journal of Psychiatry, included both identical and fraternal twins. Identical twins share all of their DNA while fraternal twins share roughly half of their genes, making them no more genetically similar than non-twin siblings.

If genes are a more important factor than shared environment in a given disorder, then identical twins would be more similar in their risk of the problem than fraternal twins would be.

Mataix-Cols and his colleagues found that among female identical twins, when one twin showed compulsive hoarding symptoms, the other twin also did 52 percent of the time. Among fraternal twins, that figure was 27 percent. There was no evidence, however, that environmental factors shared by twins contributed to compulsive hoarding. Instead, “non-shared” environmental factors — those unique to individuals — seemed to be at work.

Past research has shown that many people with hoarding problems have a history of traumatic events, according to Mataix-Cols. In particular, they have elevated rates of sexual abuse and “loss” — of a loved one or a home, for instance.

“What the study suggests is that genes are important, but probably some environmental stressors are needed to cause or trigger the hoarding problem,” said Mataix-Cols, adding more research is needed into this topic.

He said the hope was to find better therapies for compulsive hoarding as behavioral therapy and antidepressants are now the main forms of treatment, but they have met with limited success.